Direct fired heaters find wide application, particularly in oil refineries, where they are used for the purpose of preheating petroleum or petroleum derived feed-stocks for further processing to produce such products as fuel gas, gasoline, diesel fuel, heavy fuel oil and coke. The feed-stocks are of variable composition and boiling range and require that they be preheated to varying temperatures for further processing. Some of the applications considered would be as follows:
Delayed Coking Heater Service, which is one focus of the subject invention, and which involves preheating of high boiling point feed-stocks to high temperature and transferring the heater effluent to a coke drum where it is held for a period of time during which the effluent is converted to a product slate consisting of fuel gas, low boiling point liquids, high boiling point liquids and coke.
Direct fired heaters in this service operate at the most stringent conditions of any in oil refinery service, with the exception of direct fired heaters in thermal cracking service. Thus, the design strategies applicable to heaters in delayed coking service should be applicable to other services as well, including;
Crude Heater Service, wherein pretreated as-received crude is preheated to high temperature prior to being introduced into an atmospheric distillation column where a large spectrum of products with large differences in boiling point are separated from one another, such as gasoline, diesel fuel, heavy fuel oil, and a very high boiling point residuum.
Vacuum Heater Service, wherein residuum from atmospheric distillation is preheated prior to being processed in a distillation tower operated under vacuum, to separate such products as lower boiling point liquids and very high boiling point bottoms liquids from one another.
Visbreaking Heater Service, wherein high boiling point feed-stocks are subject to heat treatment in a fired heater at temperatures lower than those used in a delayed coking heater, resulting in a product slate consisting of fuel gas, gasoline and heavy fuel oil. Reboiling Heater Service, wherein relatively low boiling point feed-stocks are preheated to temperatures at which permit separation of the feedstock constituents in a distillation column is made possible.
Fully Integrated Steam Generating-Steam Superheating-Boiler Feed-Water Service, the direct fired heater for which is the second focus of the subject invention.
The direct fired heaters used for the above services are usually provided with two sections, a radiant section and a convection section. The radiant section consists of a refractory lined enclosure wherein is disposed one or more tubular heating coils thru which the process fluid flows. The heating coils are arranged so as to surround a grouping of one or more burners fueled by gas. The heating coils are arranged so as to form a combustion chamber into which high temperature combustion products generated by the burners are discharged. Heat is transferred from the combustion products to the heating coils, and the process fluid which they contain, principally by radiation.
Process fluid is usually preheated in a convection section prior to entering the radiant section, the convection section consisting of a refractory lined enclosure containing multiple rows of tubes, the rows and the tubes comprising the rows are closely spaced, forming channels thru which combustion products, leaving the radiant section, pass at relatively high velocity. In so doing, heat is transferred from the combustion products to the heating coils and contained process fluid, principally by convection. Ideally, the spent combustion products leave the convection section at low temperature corresponding to a high overall heater thermal efficiency.
Because of the high temperature to which hydrocarbon process fluids in the radiant section are subjected, fluid at the inside wall of the tubular heating elements at this location experience a degree of thermal decomposition, leaving behind adherent coke deposits which reach maximum thickness at the outlet of the coil. These deposits restrict the flow of heat from the tube wall to the contained process fluid so that the tube wall eventually reaches design temperature. At this point, referred to as an end of run condition, the heater must be shut down and de-coked to avoid tube damage. The time interval between shutdowns for decoking is referred to as run length.